Method for optimizing implant designs

ABSTRACT

Methods are disclosed for designing a tibial implant to minimize cortical impingement of a keel or other fixation structure when the tibial implant is implanted in the tibia bone. The design of the keel or other fixation structure on the tibial baseplate can be based on determining a common area between defined cancellous regions of at least two tibia bones. Methods are disclosed for designing a femoral component having a stem extension such that the stem can be sufficiently placed in the diaphysis of the femur when the femoral component is implanted. The method includes determining a canal axis in a femur that creates adequate engagement between a reamer and the diaphysis of the femur.

CLAIM OF PRIORITY

This application is a continuation of U.S. patent application Ser. No.15/890,735, filed on Feb. 7, 2018, which is a divisional of U.S. patentapplication Ser. No. 14/471,440, filed on Aug. 28, 2014, now issued asU.S. Pat. No. 9,925,052, which claims the benefit of priority under 35U.S.C. § 119(e) of U.S. Provisional Patent Application Ser. No.61/872,296, filed on Aug. 30, 2013, each of which are incorporated byreference herein in its entirety.

TECHNICAL FIELD

The present patent application relates to orthopedic prostheses, andmore particularly, to methods and systems for designing femoral andtibial components of a knee prosthesis.

BACKGROUND

Orthopedic prostheses are commonly utilized to repair and/or replacedamaged bone and tissue in the human body. For a damaged knee, a kneeprosthesis may be implanted using a proximal tibial baseplate component,a tibial bearing component, and a distal femoral component. The tibialbaseplate component is affixed to a proximal end of the patient's tibia,which is typically resected to accept the baseplate component. Thefemoral component is implanted on a distal end of the patient's femur,which is also typically resected to accept the femoral component. Thetibial bearing component is placed between the tibial baseplatecomponent and the femoral component, and may be fixed or slidablycoupled to the tibial baseplate component.

The tibial baseplate component provides support for the tibial bearingcomponent. Forces generated by use of the knee prosthesis aretransferred through the tibial bearing component to the tibial baseplatecomponent, and ultimately to the tibia. In order to ensure long termperformance of the knee prosthesis, stable and firm securement of thetibial baseplate component to the proximal end of the patient's tibia isdesired. The tibial baseplate can include securement features, such askeels or pegs, which can improve fixation of the tibial component to theproximal end of the tibia.

The femoral component replaces the articular surfaces of one or both ofthe natural femoral condyles and can articulate with the tibial bearingcomponent. A stem extension can be attached to or a part of the femoralcomponent and can be configured for insertion in the femoral canal. Thestem extension can provide stability for securement of the femoralcomponent to the distal end of the femur.

OVERVIEW

The present inventors have recognized, among other things, anopportunity for improved placement of a keel, or other fixationstructure, on a tibial baseplate in order to minimize impingement of thekeel with an inner cortical surface of a patient's tibia after thetibial baseplate is secured to the tibia. The present inventors haverecognized, among other things, an opportunity for improved placement ofa femoral stem extension on a femoral component.

To better illustrate the systems and methods disclosed herein, thefollowing non-limiting examples are provided:

In Example 1, a method of designing a tibial implant to minimizecortical impingement in a metaphyseal region of a tibia bone when thetibial implant is implanted in the tibia bone can include determining areference point with respect to a location on a tibial tray of aprovisional implant and placing the provisional implant on at least twotibia bones. The method can further include defining a two-dimensionalcancellous region enclosed by metaphyseal cortex for each of the atleast two tibia bones and determining a common area between the definedcancellous regions of the at least two tibia bones at the cross-sectionof a specific proximal-distal location on the at least two tibia bones.The method can further include determining a desired target regionwithin the cancellous region of the at least two tibia bones forplacement of a fixation structure of the tibial implant, and fabricatingan implant comprising a tibial tray and a fixation structure configuredto be attached to the tibial tray. With this method, the implant isconfigured such that, when implanted in a tibia bone having a comparableoverall size to the at least two tibia bones, the fixation structure canbe located within the desired target region for placement.

In Example 2, the method of Example 1 can optionally be configured suchthat defining the cancellous region enclosed by metaphyseal cortexincludes determining at least three points on each of the at least twotibia bones bone that define an inner cortical surface of each of the atleast two bones.

In Example 3, the method of Example 1 or 2 can optionally be configuredsuch that determining the common area between the defined cancellousregions includes mapping the at least three points on each of the atleast two tibia bones onto the two-dimensional coordinate system tocreate a polygon-shaped area for each of the at least two tibia bones.

In Example 4, the method of Example 3 can optionally be configured suchthat determining the common area further includes overlaying thepolygon-shaped areas for each of the at least two tibia bones to findthe common area.

In Example 5, the method of any of Examples 1-4 can optionally beconfigured such that the desired target keel region is defined by thecommon area subtracted by a radius of the fixation structure or adimension of an instrument used in implanting the tibial implant.

In Example 6, a method of determining a canal axis that creates a bestfit between a reamer and a diaphysis of a femur for preparing the femurfor a femoral implant can include providing two or more cylinders havingvarious diameters and representing reamers configured for use inpreparing the femur for the femoral implant, and inserting a firstcylinder, having a first diameter, into a canal of the femur to apredetermined reaming depth. If the reaming depth achievable with thefirst cylinder is less than the predetermined reaming depth, the methodcan include inserting the first cylinder into the canal to a maximumreaming depth that the first cylinder is able to fit in the canal. Ifthe reaming depth achievable is about equal to or greater than thepredetermined reaming depth and the first cylinder is not seated againstan inner cortex of the femur, the method can include inserting a secondcylinder, having a second diameter, into the canal of the femur to thepredetermined reaming depth. The second diameter can be greater than thefirst diameter of the first cylinder. Cylinders of increasingly greaterdiameters can be inserted into the canal of the femur until a particularcylinder is seated in the canal. The method can further includedetermining an optimal cylinder position and an optimal canal axis canbe based on a longitudinal axis of the optimal cylinder position.

In Example 7, the method of Example 6 can optionally further includeadjusting an entry point for inserting the particular cylinder in thedistal end of the femur in at least one of an anterior/posteriordirection and a medial/lateral direction.

In Example 8, a method of designing a femoral component having a stemextension and configured for implantation on a distal end of a femur caninclude determining a canal axis of a plurality of femurs using at leastone cylinder to create an optimal fit between a reamer and a diaphysisof each of the plurality of femurs. The plurality of femurs can have acomparable size and can be configured to correspond to one implant sizeof an implant family. The method can further include determining aposition of the stem extension on the femoral component as a function ofthe determined canal axis of the plurality of femurs.

In Example 9, the method of Example 8 can optionally be configured suchthe position of the stem extension on the femoral component can bedetermined for each of the plurality of femurs and averaged to determinethe position of the stem extension on the femoral component for aparticular size in the implant family.

This overview is intended to provide an overview of subject matter ofthe present patent application. It is not intended to provide anexclusive or exhaustive explanation of the invention. The detaileddescription is included to provide further information about the presentpatent application.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings, which are not necessarily drawn to scale, like numeralsmay describe similar components in different views. Like numerals havingdifferent letter suffixes may represent different instances of similarcomponents. The drawings illustrate generally, by way of example, butnot by way of limitation, various embodiments discussed in the presentdocument.

FIG. 1 is a top view of a proximal end of an example of a tibialbaseplate.

FIG. 2 is an anterior side view of the tibial baseplate of FIG. 1.

FIG. 3 is a bottom view of a distal end of another example of a tibialbaseplate.

FIG. 4 is a lateral side view of the tibial baseplate of FIG. 3.

FIG. 5 is a top view of an outer periphery of an example of aprovisional implant and its use in creating a two dimensional coordinatesystem.

FIG. 6 is a plot of the two dimensional coordinate system illustrating adetermined ‘sweet spot’ for keel placement.

FIG. 7A is a plot of the x-values that form the sweet spot for each ofnine implant sizes.

FIG. 7B is a plot of the y-values that correspond to the x-values ofFIG. 7A.

FIG. 8A is a side view of an example of a femoral component having astem extension.

FIG. 8B is an posterior view of the femoral component of FIG. 8A.

FIG. 9A is an anterior view of a femur with a cylinder inserted in thecanal of the femur.

FIG. 9B is a side view of the femur and cylinder of FIG. 9A.

FIG. 9C is a distal view of the femur and cylinder of FIG. 9A.

FIG. 10A is an anterior view of a femur with a cylinder inserted in thecanal of the femur.

FIG. 10B is a side view of the femur and cylinder of FIG. 10A.

FIG. 10C is a distal view of the femur and cylinder of FIG. 10A.

FIG. 11A is a side view of a femur with a cylinder inserted in the canalof the femur.

FIG. 11B is a distal view of the femur and cylinder of FIG. 11A.

FIG. 12A is a side view of a femur with a cylinder inserted in the canalof the femur.

FIG. 12B is a distal view of the femur and cylinder of FIG. 12A.

FIG. 13 is a side view of a femur with a cylinder inserted in the canalof the femur.

FIG. 14A is a plot of an anterior offset as a function of femur size asmeasured for individual bones.

FIG. 14B is a plot of an anterior offset of a femoral stem on thefemoral component as a function of femoral implant size.

DETAILED DESCRIPTION

The present application relates to systems and methods for designing atibial baseplate with a fixation structure, such as a keel or a peg,configured to minimize impingement when implanted on a tibia. The tibialbaseplate can be part of a knee prosthesis and the fixation structurecan promote securement and/or stabilization of the tibial baseplate to apatient's proximal tibia. The present application relates to systems andmethods for designing a femoral prosthesis component that can include afemoral stem. The femoral component can be part of a knee prosthesis andthe femoral stem can promote securement and/or stabilization of thefemoral component to a patient's distal femur, as part of, for example,a revision procedure.

A patient's tibia and/or femur can be prepared to receive the prosthesiscomponent of the subject matter disclosed herein, by way of any suitablemethod or apparatus known by one of skill in the art. A surgical methodcan involve resection of the distal end of a patient's femur and/orresection of the proximal end of the patient's tibia. A method ofresection can include forming a substantially planar resected surface ofthe femur and/or tibia. For a femoral component that includes a femoralstem, the surgical method can include preparation of the femoralintramedullary canal, through reaming, for receiving the femoral stem.

The term “proximal,” as used herein, refers to the direction generallytoward the torso of a patient. The term “distal,” as used herein, refersto the direction generally away from the torso of a patient, or in theopposite direction of proximal. “Anterior,” as used herein, refers tothe general direction toward the front of a patient or a knee. As usedherein, “posterior” generally refers to the direction toward the back ofa patient or knee (the opposite direction of anterior). As used herein,“lateral” refers to the general direction away from the middle of thepatient, and away from the sagittal plane. “Medial,” as used hereinrefers to the general direction toward the middle of the patient and thesagittal plane, (the opposite direction of lateral). When used inreference to a knee, the term “lateral” refers to the general directionaway from the patient's other knee, while “medial” refers to the generaldirection toward the patient's other knee.

With reference to the figures, some anatomical regions are labeled forclarity. In some figures, the anterior region of a tibia is labeled “A,”the posterior region “P,” the lateral region “L,” and the medial region“M.” In some figures, the anterior/lateral region of a tibia is labeled“AL,” the posterior/lateral region is labeled “PL,” the posterior/medialregion is labeled “PM,” and the anterior/medial region is labeled “AM.”

Right and left knee prosthesis configurations are mirror images of oneanother about a sagittal plane. Therefore, regardless of theconfiguration depicted herein, is will be appreciated that the aspectsof the prosthesis described are equally applicable to a right knee orleft knee prosthesis.

1. Design of Tibial Baseplate. Including Placement of Fixation SupportStructure(s)

FIG. 1 shows a proximal end of an example of a tibial baseplate 100 thatcan be used as a component in a knee prosthesis. The tibial baseplate100 can include a proximal surface 111 configured to receive a tibialbearing component in a fixed or a sliding relationship. In an example,the baseplate 100 can include a raised rim 113 around the proximalsurface 11 to receive surround, and hold the tibial bearing componenttherein; alternative structures can be used to receive and hold thetibial bearing component. The tibial bearing component can be configuredto interact with a patient's distal femur or a femoral prosthesiscomponent.

The tibial baseplate 100 can include an outer periphery 112, which canhave a thickness T in a proximal/distal direction (see FIG. 2). Theouter periphery 112 of the tibial baseplate 100 can form part of thetibial baseplate 100 referred to as the tibial tray. The outer periphery112 can be defined by an anterior face 118, a posterior/lateral face120, a posterior/medial face 122, a PCL cutout area 124, a lateral face162 and a medial face 160. The tibial baseplate 100 can also include alateral compartment 114, a medial compartment 116, and ananterior-posterior home axis A_(H) separating the lateral 114 and medial116 compartments. The anterior face 118 can include a linear or flatportion 118 a that can be generally central located between lateral 114and medial 116 compartments. The flat portion 118 a can define ananterior-most extent of the tibial baseplate 100. As shown in FIG. 1,the tibial baseplate 100 can be side specific and can be asymmetric,such that the lateral 114 and medial 116 compartments can be differentin size and/or shape.

FIG. 2 shows the side of the tibial baseplate 100, which can include asingle fixation structure, a keel 130, which can extend distally from adistal surface 134 of the tibial baseplate 100 and into a tibia. Thekeel 130 can be monolithically or integrally formed as part of thetibial baseplate 100 or the keel 130 can be separately attachable to thedistal surface 134 of the tibial baseplate 100. The keel 130 can have acylindrical core 131 defining a longitudinal axis A_(K) and having twoor more fins 133 extending radially outwardly therefrom, and the fins133 can be arranged symmetrically relative to the cylindrical core 131.The distal surface 134 can be the surface which contacts a resectedsurface of a patient's tibia after the baseplate 100 is implanted, atwhich point the keel 130 can extend into a cancellous region of themetaphysic or intramedullary canal of the tibia.

The keel 130 can be asymmetrically disposed on the distal surface 134with respect to the home axis A_(H). In an example, the longitudinalkeel axis A_(K) can be biased medially with respect to a vertical planethat contains home axis A_(H)—in other words, the keel axis A_(K) can beoffset toward the medial compartment 116 and away from the lateralcompartment 114 by offset distance 163. Thus, a medial distance 164between the keel axis A_(K) and the medial-most portion of the medialface 160 can be less than a lateral distance 166 between the keel axisA_(K) and the lateral-most portion of the lateral face 162.

As shown in FIG. 1, an anterior/posterior keel distance 147 can bemeasured posteriorly from the flat portion 118 a of the anterior face118 to the keel axis A_(K), for example. A lateral depth 144 of thelateral compartment 114 can be measured posteriorly from the flatportion 118 a of the anterior face 118 to the posterior/lateral face 120of baseplate 100 in FIG. 1. The lateral depth 144 can exceed keeldistance 147.

FIG. 3 shows a distal surface of an example of a tibial baseplate 200that includes two fixation structures instead of the keel 130 of thetibial baseplate 100 of FIGS. 1 and 2. The proximal surface (not shown)of the tibial baseplate 200 can be similar to the proximal surface 111of the tibial baseplate 100 and the outer periphery 212 can include thesame elements.

The tibial baseplate 200 can include a lateral fixation peg 230 and amedial fixation peg 232, that can each extend distally from a distalsurface 234 of the tibial baseplate 200 and into a cancellous region ofthe metaphysis or intramedullary canal of the tibia. In an example, thelateral 230 and medial 232 fixation pegs can be asymmetrically arrangedabout the anterior-posterior axis A_(H).

FIG. 4 shows the side of the tibial baseplate 200. In an example, thefixation pegs 230 and 232 can be hexagonal in cross-section near thedistal surface 34 and can transition to a circular cross-section as thepegs 230 and 232 extend away from the distal surface 34.

The fixation structures of the tibial baseplates 100 and 200 (the keel130 and the pegs 230 and 232, respectively) can be designed such thatwhen the tibial baseplates 100 and 200 are implanted on a resectedtibia, impingement with surrounding bone can be minimized. The presentapplication discloses systems and methods for such design of thefixation structure on the tibial baseplate. Although the tibialbaseplate 100 includes a keel 130 and the tibial baseplate 100 includestwo fixation pegs 230 and 232, it is recognized that additional oralternative fixation structures can be use with a tibial baseplate andare within the scope of the present application. For example, instead oftwo fixation pegs, a tibial baseplate can be designed to have fourfixation pegs.

Reference is made to a co-pending application, U.S. Ser. No. 13/593,339,Publication No. US 2013/0131820, and entitled “TIBIAL BASEPLATE WITHASYMMETRIC PLACEMENT OF FIXATION STRUCTURES,” for further disclosure onthe tibial baseplate and the fixation structures described herein.

A method is described below for designing a tibial implant to minimizecortical impingement in a metaphyseal region when a tibial implant isimplanted on a tibia. In an example, the method can be used to determinea “sweet spot” or area for placing the fixation structure on the tibialtray such that, for that particular implant size, cortical impingementcan be reduced or eliminated.

The method can include creating a coordinate system that corresponds toa location on a tibial baseplate for each size implant in a tibialimplant family. FIG. 5 shows an example of a provisional baseplate 300having an outer periphery that is shaped similarly to the tibialbaseplates 100 and 200 described above. A reference point 302 on theprovisional baseplate 300 can be used as an origin for the coordinatesystem. In an example, the reference point 302 can be based on ananterior/posterior position on the provisional baseplate 300 and amedial/lateral position on the provisional baseplate 300. The referencepoint 302 can be based on an intersection of (1) a first mid-pointbetween a medial-most point 304 and a lateral-most point 306 on theprovisional baseplate 300, and (2) a second mid-point between ananterior-most point 308 and a posterior-most point 310 on theprovisional baseplate 300. The reference point 302 can be the origin foran x-y coordinate system.

As described further below, a ‘sweet spot’ 303 can be created by placingthe provisional baseplate 300 on at least two resected tibia bones,measuring three points on the resected tibia and then plotting thosethree points for each measured bone using the reference point 302 on theprovisional baseplate 300. (Note that the ‘sweet spot’ 303 of FIG. 5 isexemplary and is not based on actual data.) The three points cancorrespond to a two-dimensional cancellous region of the tibia bone at aspecific proximal-distal location. The cancellous region, which can beenclosed by metaphyseal cortex, can be used to determine the innercortical surface of the tibia bone, which can define the cancellousregion of the metaphysis or intramedullary canal.

The three data points can result in formation of a triangle in whicheach bone has an x1,y1 data point, an x2,y2 data point, and an x3,y3data point. By plotting the three data points for each bone, and forminga triangle for each bone, a common area or ‘sweet spot’ between thetriangles (like the exemplary sweet spot 303 of FIG. 5) can bedetermined. In other examples, more than three data points can be used,in which case the mapped data points can create another type of polygonshape instead of the triangle shown in FIG. 5.

This method can be done for each size of a tibial baseplate—at least twobones that correspond to each tibial baseplate size can be measured andthe data plotted to determine the ‘sweet spot’. These measurements canbe performed on numerous bones. In an example, forty to fifty bones canbe measured for each tibial baseplate size. In other examples, more orless bones can be measured for each tibial baseplate size. Themeasurements can be performed on actual bones and/or using digital bonedata.

FIG. 6 shows the provisional baseplate 300 of FIG. 5 and the coordinatesystem described above, as well as an example of a triangle or ‘sweetspot’ 305 formed for one size of a tibial baseplate. The ‘sweet spot’305 can be used to determine where a keel can be implanted in thecancellous region of the metaphysic or intramedullary canal to minimizeimpingement, and in turn, where the keel can be located on the tibialtray of the provisional baseplate 300, which can correspond to thekeel's location on a tray of the tibial implant. By finding a commonarea for multiple bones that correspond to the same size tibialbaseplate, if the placement of the keel of the tibial tray is based onthe common area, impingement can be minimized or reduced when the tibialbaseplate is implanted.

Moreover, because the common area or sweet spot can be based on multipleresected bones, the common area can also include resected tibias havingsurgical variability among them. For example, the multiple resectedbones can include bones having varying degrees of resection slope and/orbones having varying degrees of implant placement rotation and overhang.Thus this methodology can result in minimizing or reducing impingementeven in the presence of surgical variability.

In an example, the sweet spot can be a region where a center point onthe keel can be placed and have little to no impingement. Since thissweet spot can be based on measurements taken, for example, at thecortex of the bone, the sweet spot can potentially place the centerpoint of the keel near the cortex in some instances, in which case theexternal surfaces of the keel could cause some impingement at thatregion. Thus the desired target region for placement of the keel can bebased on the common area and adjusted to account for the overall sizeand shape of the keel, which can be used in designing the keel'slocation on the tibial tray. The sweet spot or common area can bereduced by a radius or other characteristic dimension of the keel (orother fixation structure). In an example, instrumentation can be usedduring implantation—such as a broach—which can be larger in size thanthe keel, in which case the common area can be reduced by the size ofsuch instrumentation.

FIGS. 7A and 7B show examples of data collected using the methoddescribed above. FIG. 7A shows the three x values (x1, x2, x3) for eachof nine implant sizes, and FIG. 7B shows the three y values (y1, y2, y3)for the nine implant sizes of FIG. 7A. When the three data points(x1,y1; x2,y2; x3,y3) are plotted for each implant size, it represents asweet spot for each implant size. The values shown in FIGS. 6, 7A and 7Brepresent a set of exemplary data. It is recognized that different datawill be collected depending on, for example, how and where themeasurements are taken on the bone, and the configuration of the tibialbaseplate that the fixation structure is intended to be used with.

The method described above offers a design of a tibial baseplate inwhich a keel, or other fixation structure, can be located on the tibialbaseplate in such a way that the fixation structure will have little tono impingement with the cortical bone. Due to the methodology used todetermine the sweet spot for locating the keel or other fixationstructures on the tibial baseplate, the keel design can accommodatesurgical variability, such as, for example, a range of resection slopes.

The method was described in the context of a tibial baseplate having akeel, similar to a design of the tibial baseplate 100 of FIGS. 1 and 2.The method can also be used for a tibial baseplate having alternativefixation structures, such as for example, the two pegs 230 and 232 ofthe tibial baseplate 200. The methodology can be generallysimilar—measurements can be taken using tibial baseplates and at leasttwo bones that correspond to each implant size. For the two pegs 230 and232, two ‘sweet spots’ or common areas for each implant size can bedetermined—each sweet spot can correspond to one of the pegs 230 and232.

2. Design of Femoral Component, Including Stem Housing

FIGS. 8A and 8B show an example of a femoral component 400 that can beused in a knee prosthesis. The femoral component 400 can be specific toeither a right or left leg. The femoral component 400 can includecondyles 402 that can be configured to articulate with a proximal end ofa tibia bone or a tibial implant. The femoral component 400 can includea stem 406 that can extend from a non-articulating portion of thefemoral component 400 and can be configured for placement in a canal ofa femur. The use of a femoral stem, like femoral stem 406, andsufficient placement of the stem in the canal can improve the fixationstability from a knee arthroplasty, including a revision total kneeprocedure.

A method is described for designing a femoral stem on a femoralcomponent using a plurality of femoral bones.

A. Analyzing Femoral Canal to Determine a Reamer/Canal Axis

A reamer can be used to prepare a femoral canal for receiving the stem406 when the femoral component 400 is implanted on the distal end of thefemur. Deep reaming of the canal can be important for achievingdiaphyseal engagement, especially when the femoral stem 406 isconfigured for a press-fit. In an example, it can be desirable to reamto a depth of about 200 mm.

FIGS. 9A and 9B show a femur bone 500 with a cylinder 502 (which canrepresent a reamer) inserted into a diaphysis 504 of the femur 500. FIG.9C shows a distal end 506 of the femur 500 and an entry location of thecylinder 502 on the distal end 506. In an example, as shown in FIGS.9A-9C, the cylinder 502 can have a diameter of about 10 mm.

To determine the reamer/canal axis, a method can be performed onmultiple bones. The method can be performed virtually, for example,using a digital library of bones, or it can be performed manually usingfemur bones and one or more cylinders having various diameters andrepresenting reamers configured for use in preparing the femur for thefemoral implant. First, a user can create a best fit between a cylinderand the diaphysis of the femur, up to a particular predetermined reamingdepth, such that the cylinder can be seated against the inner cortex andcan be held in place. In an example, the predetermined reaming depth canbe about 200 mm. In an example, the predetermined reaming depth can beabout 150 mm. Other depths less than 150 mm and greater than 200 mm canbe used as the predetermined reaming depth.

In the method described herein, the predetermined reaming depth is 200mm. The method can include maximizing a diameter of a cylinder tocontact the endosteal walls of the canal up to the 200 mm reaming depth.In an example, if a smallest reamer in a set of reamers has a diameterof 10 mm, the smallest cylinder in the set of one or more cylinders canhave a diameter less than or about 10 mm. If the smallest cylinder isinserted into the canal and the reaming depth of 200 mm is not achieved,the method can include reducing the reaming depth of the cylinder untilthe cylinder can fit in a canal of the femur. Once the cylinder isproperly seated at a maximum ream depth achievable, the optimal cylinderposition can be determined.

On the other hand, if the smallest cylinder (in an example, 10 mmdiameter cylinder) can achieve a reaming depth of about 200 mm orgreater and the cylinder is not seated against an inner cortex of thefemur or does not have sufficient contact with the walls of the canal atthe reaming depth, a second cylinder having a greater diameter can beinserted into the canal, up to a reaming depth of about 200 mm.Cylinders having increasingly greater diameters can be inserted into thecanal until sufficient seating or orientation of the cylinder in thecanal is achieved. In an example, the cylinders can have increasinglygreater diameters at 1 mm increments.

The method of determining the reamer/canal axis can include adjusting anentry point for inserting the particular cylinder in the distal end ofthe femur. The entry point can be adjusted in an anterior/posteriordirection and/or a medial/later direction. In an example, the entrypoint can move within a square location 0-10 mm anterior and ±5 mmmedial/lateral to a center of the femur, such that the square can createthe boundaries of the entry point region. The entry point can beadjusted, for example, to account for different bone shapes. The methodcan also include having two or more users verify the appropriateness ofthe determined reamer/canal axis during a surgical procedure. Using themethod steps above, a user can determine a reamer/canal axis whichdetermines where the reamer or cylinder enters into the femur at thedistal end.

FIGS. 10A-10C show a femur bone 600 with a cylinder 602 having adiameter of about 11 mm inserted into a diaphysis 604 of the femur 600up to a depth of about 170 mm. A narrowing of the diaphysis or canal 604of the femur 600 can prohibit a minimum 10 mm cylinder at a 200 mmreaming depth as performed in the femur 500 of FIGS. 9A and 9B. Thus alarger diameter cylinder 602 can be used, and the depth of insertion canbe reduced, as compared to the femur 500 of FIGS. 9A and 9B. An entrylocation of the cylinder 602 on a distal end 506 of the femur 500 isshown in FIG. 9C.

FIG. 11A shows an original reamer depth and an originally determinedentry point of a cylinder 702 into a femur 700. FIG. 12A shows anadjusted reamer depth and adjusted entry point of a cylinder 802 into afemur 800 based on an excessive anterior location caused by a femoralbow of the femur 800. FIGS. 11B and 12B illustrate a difference D1between the entry points of each of the cylinders 702 and 802 shown inFIGS. 11A and 12A, respectively. An increased reaming depth can lead toan anterior shift of the distal entry point.

The method of determining the reamer/canal axis can be performed onnumerous bones to collect data that can represent the variations betweenfemur bones from patient to patient. As described above, the method caninclude using various cylinders having different diameters, andadjusting the reaming depth and/or the entry point to find effectivepositioning of the cylinder.

In an example, the method of determining the canal or reamer axis caninclude virtual remaining as mentioned above, and a reaming algorithm.The algorithm can maximize certain factors and conditions, and minimizeothers. For example, the algorithm can maximize reamer diameter andoptimize engagement with the inner diaphysis of the femur. The targetreaming depth can be set to 200 mm and if that is not feasible for aparticular femur, the reaming depth can be shortened at 5 mm intervalsuntil the canal of the femur can accommodate the reamer.

B. Determining Position of Femoral Stem Housing

The method described above for determining a canal axis for reamerplacement can be used in designing a stem housing on a femoral implant.The method for designing the stem housing can include locating ananterior cortex 907 on a femur 900 (a second reference point on anoutside of the femoral bone where a saw blade cuts an anterior surfaceof the femur) and measuring a distance D2 from the anterior cortex 907to a canal/reamer axis 910 at a proximal/distal location, as shown inFIG. 13.

A distal cut plane or resection plane 914 can be used as a reference fordetermining the proximal/distal location for measuring the distance D2.In an example, the distal cut plane 914 can correspond to a distalfemoral cut of 11 mm-a 9 mm primary distal resection with about 2 mm ofbone loss. In an example, the distance D2 can be measured at a locationabout 40 mm proximal to the distal cut plane 914. The method fordetermining a position of the femoral stem housing can include applyingthe measured distance D2 to a femoral implant by locating an axis of thestem housing on the femoral component the distance D2 from an anteriorflange of the femoral component.

FIG. 13 also includes a distance D3 which corresponds to an overall bonesize and is defined as the distance between the anterior cortex 907 anda plane tangent to posterior condyles 912. The distance D3 can also bereferred to as a functional AP dimension (fAP or FAP).

Similar to the method described under Part A for the canal axis, thismethodology for the stem housing can be performed for each individualbone, including, in an example, multiple bones contained in a digitallibrary of bones. Based on the measurement gathered for each bone, alocation of the femoral stem on the femoral implant can be determined.The measurement for each bone can be used to create a patient databasefor determining a design of the stem housing that can best fit a canalof a patient's femur. The data can be averaged, based on, for example, asize of the femur and/or the femoral component.

FIG. 14A shows the anterior/posterior (AP) distance as a function ofoverall bone size (functional AP) for bones of different sizes. (SeeFIG. 13 defining these dimensions.) Since the bone size determines theimplant size, this data can be used to determine the offset in design ofthe femoral stem. FIG. 14B shows the average measurement of a housingoffset of the femoral stem for various sizes of femoral components inthe implant family—larger offset values correspond to a more posteriorposition on the femoral component. Thus the design of the femoral stem,and its location on the femoral component, can be tailored for eachfemoral implant size. Because the collected data is based on an averageof measurements collected on individual bones, in designing the femoralstem, the designer can select a location within the band shown in FIG.14B of the average value plus or minus one standard deviation. As shownin FIG. 14B, as the implant size increases (to correspond to a largerfemur size), the position of the stem on the femoral component is moreposterior. Thus the femoral implants can be designed such that theanatomical stem housing can be positioned more anterior for smallerfemurs and more posterior for larger femurs.

By using data collected on multiple bones and grouping the data byimplant size, the method described herein can facilitate optimal designof the femoral stem on the femoral implant such that the stem can beadequately implanted in the canal of the patient's femur.

The above detailed description includes references to the accompanyingdrawings, which form a part of the detailed description. The drawingsshow, by way of illustration, specific embodiments in which theinvention can be practiced. These embodiments are also referred toherein as “examples.” Such examples can include elements in addition tothose shown or described. However, the present inventors alsocontemplate examples in which only those elements shown or described areprovided. Moreover, the present inventors also contemplate examplesusing any combination or permutation of those elements shown ordescribed (or one or more aspects thereof), either with respect to aparticular example (or one or more aspects thereof), or with respect toother examples (or one or more aspects thereof) shown or describedherein.

In the event of inconsistent usages between this document and anydocuments so incorporated by reference, the usage in this documentcontrols. In this document, the terms “a” or “an” are used, as is commonin patent documents, to include one or more than one, independent of anyother instances or usages of “at least one” or “one or more.” In thisdocument, the term “or” is used to refer to a nonexclusive or, such that“A or B” includes “A but not B,” “B but not A,” and “A and B,” unlessotherwise indicated. In this document, the terms “including” and “inwhich” are used as the plain-English equivalents of the respective terms“comprising” and “wherein.” Also, in the following claims, the terms“including” and “comprising” are open-ended, that is, a system, device,article, composition, formulation, or process that includes elements inaddition to those listed after such a term in a claim are still deemedto fall within the scope of that claim. Moreover, in the followingclaims, the terms “first,” “second,” and “third,” etc. are used merelyas labels, and are not intended to impose numerical requirements ontheir objects.

Method examples described herein can be machine or computer-implementedat least in part. Some examples can include a computer-readable mediumor machine-readable medium encoded with instructions operable toconfigure an electronic device to perform methods as described in theabove examples. An implementation of such methods can include code, suchas microcode, assembly language code, a higher-level language code, orthe like. Such code can include computer readable instructions forperforming various methods. The code may form portions of computerprogram products. Further, in an example, the code can be tangiblystored on one or more volatile, non-transitory, or non-volatile tangiblecomputer-readable media, such as during execution or at other times.Examples of these tangible computer-readable media can include, but arenot limited to, hard disks, removable magnetic disks, removable opticaldisks (e.g., compact disks and digital video disks), magnetic cassettes,memory cards or sticks, random access memories (RAMs), read onlymemories (ROMs), and the like.

The above description is intended to be illustrative, and notrestrictive. For example, the above-described examples (or one or moreaspects thereof) may be used in combination with each other. Otherembodiments can be used, such as by one of ordinary skill in the artupon reviewing the above description. The Abstract is provided to complywith 37 C.F.R. § 1.72(b), to allow the reader to quickly ascertain thenature of the technical disclosure. It is submitted with theunderstanding that it will not be used to interpret or limit the scopeor meaning of the claims. Also, in the above Detailed Description,various features may be grouped together to streamline the disclosure.This should not be interpreted as intending that an unclaimed disclosedfeature is essential to any claim. Rather, inventive subject matter maylie in less than all features of a particular disclosed embodiment.Thus, the following claims are hereby incorporated into the DetailedDescription as examples or embodiments, with each claim standing on itsown as a separate embodiment, and it is contemplated that suchembodiments can be combined with each other in various combinations orpermutations. The scope of the invention should be determined withreference to the appended claims, along with the full scope ofequivalents to which such claims are entitled.

The claimed invention is:
 1. A method of designing a tibial implant tominimize cortical impingement in a metaphyseal region of a tibia bonewhen the tibial implant is implanted in the tibia bone, the methodcomprising: determining a reference point with respect to a location ona tibial tray of a provisional implant; placing the provisional implanton at least two tibia bones; defining a two-dimensional cancellousregion enclosed by metaphyseal cortex for each of the at least two tibiabones; determining a common area between the defined cancellous regionsof the at least two tibia bones at the cross-section of a specificproximal-distal location on the at least two tibia bones; determining adesired target region within the cancellous region of the at least twotibia bones for placement of a fixation structure of the tibial implant;and fabricating an implant comprising a tibial tray and a fixationstructure configured to be attached to the tibial tray, wherein theimplant is configured such that, when implanted in a tibia bone having acomparable overall size to the at least two tibia bones, the fixationstructure is located within the desired target region for placement. 2.The method of claim 1, wherein defining the cancellous region enclosedby metaphyseal cortex includes determining at least three points on eachof the at least two tibia bones bone that define an inner corticalsurface of each of the at least two bones.
 3. The method of claim 2,wherein the determined reference point provides an origin for atwo-dimensional coordinate system at the cross-section of the specificproximal-distal location, and determining the common area between thedefined cancellous regions includes mapping the at least three points oneach of the at least two tibia bones onto the two-dimensional coordinatesystem to create a polygon-shaped area for each of the at least twotibia bones.
 4. The method of claim 3, wherein determining the commonarea further includes overlaying the polygon-shaped areas for each ofthe at least two tibia bones to find the common area.
 5. The method ofclaim 1, wherein the at least two tibia bones have been resected atdifferent angles.
 6. The method of claim 5, wherein a resection angle ofeach of the at least two tibia bones has a posterior slope between 0 and10 degrees.
 7. The method of claim 5, wherein a resection angle of eachof the at least two tibia bones ranges between varus 3 degrees andvalgus 3 degrees.
 8. The method of claim 1, wherein the implant isconfigured to have an overhang of about 1 mm or less when implanted on atibia bone.
 9. The method of claim 1, wherein the desired target keelregion is defined by the common area subtracted by a radius of thefixation structure or a dimension of an instrument used in implantingthe tibial implant.
 10. The method of claim 1, wherein the fixationstructure is a keel.
 11. The method of claim 1, wherein the fixationstructure is a peg.
 12. A method of designing a femoral component havinga stem extension and configured for implantation on a distal end of afemur, the method comprising: determining a canal axis of a plurality offemurs using at least one cylinder to create an optimal fit between areamer and a diaphysis of each of the plurality of femurs, the pluralityof femurs having a comparable size and configured to correspond to oneimplant size of an implant family; and determining a position of thestem extension on the femoral component as a function of the determinedcanal axis of the plurality of femurs.
 13. The method of claim 12,wherein the position of the stem extension on the femoral component isdetermined by measuring a distance between the canal axis and ananterior cortex in a distal cut plane for each of the plurality offemurs.
 14. The method of claim 12, wherein the position of the stemextension on the femoral component can be determined for each of theplurality of femurs and averaged to determine the position of the stemextension on the femoral component for a particular size in the implantfamily.
 15. The method of claim 12, wherein determining the canal axisof the plurality of femurs includes adjusting a distal entry point forinsertion of the at least one cylinder.
 16. The method of claim 12,wherein determining the canal axis of the plurality of femurs includes:(a) providing two or more cylinders having various diameters andrepresenting reamers configured for use in preparing a femur for thefemoral implant; (b) inserting a first cylinder, having a firstdiameter, into a canal of the femur to a predetermined reaming depth;(c) if the reaming depth achievable with the first cylinder is less thanthe predetermined reaming depth, inserting the first cylinder into thecanal to a maximum reaming depth that the first cylinder is able to fitin the canal; and (d) if the reaming depth achievable is about equal toor greater than the predetermined reaming depth and the first cylinderis not seated against an inner cortex of the femur, inserting a secondcylinder, having a second diameter, into the canal of the femur to thepredetermined reaming depth, the second diameter being greater than thefirst diameter of the first cylinder; (e) repeating step (d) usingcylinders of increasingly greater diameters, until the particularcylinder is seated in the canal; and (f) determining an optimal cylinderposition based on step (c) or (e), wherein an optimal canal axis isbased on a longitudinal axis of the optimal cylinder position.
 17. Themethod of claim 16, wherein step (f) of determining the optimal cylinderposition includes adjusting an entry point for inserting the particularcylinder in the distal end of the femur in at least one of ananterior/posterior direction and a medial/lateral direction.
 18. Themethod of claim 16, wherein repeating step (c) includes reducing thereaming depth by increments of 5 mm.
 19. The method of claim 16, whereinrepeating step (e) includes increasing a cylinder diameter by incrementsof 1 mm.
 20. The method of claim 16, wherein the diameter of the firstcylinder is 10 mm.